The present invention relates generally to gas generating systems and, more particularly, to gas generators used in vehicle occupant protection systems and related components, such as seatbelt pretensioners.
Gas generators used in seatbelt pretensioners are known as micro gas generators due to the relatively small size of the gas generator. Exemplary pretensioners using such micro gas generators include those described in U.S. Pat. Nos. 6,460,794, 6,505,790, 6,520,443, and 6,419,177, incorporated herein by reference. Micro gas generators generally contain an initiator including an initiator charge, a header or initiator holder in which the initiator is received and secured therein, and a gas generant composition which ignites and burns in response to ignition of the initiator to produce gases for actuating the seatbelt pretensioner.
The gas generant composition must be positioned so as to enable ignition thereof by the initiator upon activation of the micro gas generator. In typical micro gas generator designs, a casing containing the gas generant composition is hermetically attached to the initiator holder, with the gas generant composition in direct fluid communication with a casing containing the initiator charge. Upon activation of the initiator, by-products from initiator charge combustion fracture or otherwise penetrate the initiator charge casing, igniting the gas generant. However, the need to provide features enabling hermetic attachment of the gas generant casing to the initiator holder complicates the design of the holder. For example, crimp tabs or flanges formed in the holder for accommodating fasteners for casing attachment, as well as grooves and surfaces formed along the holder to accommodate compliant seals for hermetically encapsulating the gas generant, increase the complexity and cost of fabricating the initiator holder. Furthermore, the provision of sealing elements (for example, O-rings or other gaskets) increases the assembly part count, and the cost and time associated with manufacturing the assembly.
Yet another challenge with gas generant compositions that produce relatively small amounts of solids, sometimes known as “smokeless” compositions, is that not all non-metallic constituents contribute to stable ballistic performance when subjected to environmental conditioning. In fact, one fuel that is favored because of its propensity to produce all or mostly gas is nitrocellulose and derivatives thereof When combined with other gas generant constituents such as an oxidizer, and formed into a gas generant composition, this fuel contributes to greater amounts of gas upon combustion of the composition. It is nevertheless believed that nitrocellulose may contribute to an unacceptably aggressive ballistic performance as measured after thermal cycling and thermal shock testing defined in SAE International Document SAE/USCAR-24 “USCAR INFLATOR TECHNICAL REQUIREMENTS AND VALIDATION”, herein incorporated by reference.